1. Field of the Invention
The present invention relates to a technology for forming a surface protection film on a semiconductor device.
2. Description of the Related Art
A semiconductor device manufactured from a compound semiconductor can be effectively used as a high voltage-resistant device or a high-speed device due to an essential nature of compound semiconductor materials, such as a direct transition. Recently, a high electron mobility transistor (HEMT), which is one of the field effect transistors (FETs) manufactured from a nitride-based compound semiconductor, has been popular and various types of HEMTs are developed (see, for example, Japanese Patent Application Laid-Open No. 2005-129856 and Japanese Patent Application Laid-Open No. 2003-179082).
FIG. 8 is a cross section of a conventional HEMT of a gallium-nitride (GaN)-based compound semiconductor. In the HEMT shown in FIG. 8, a buffer layer 12 of GaN, a channel layer 13 of undoped-GaN, and an electron supply layer 14 of aluminum gallium nitride (AlGaN) that is thinner than the undoped-GaN are grown on a semi-insulating substrate 11 such as a sapphire substrate to form a heterojunction structure. A source electrode S, a gate electrode G, and a drain electrode D are arranged on the electron supply layer 14. A contact layer of n-type gallium nitride (n-GaN) (not shown) is formed between the source electrode S, the drain electrode D, and the electron supply layer 14 for reducing a contact resistance between the layers.
In the HEMT, two-dimensional electron gas generated right under a heterojunction between the channel layer 13 and the electron supply layer 14 is generally used as a career. In FIG. 8, a middle layer 16 of the nitride-based compound semiconductor with bandgap energy larger than that of the channel layer 13 is grown between the channel layer 13 and the electron supply layer 14. Further, a two-dimensional electron gas layer 15 with higher density than usual is formed between the channel layer 13 and the electron supply layer 14. As a result, a low-loss high-output FET is realized.
In the HEMT, when a voltage is applied between the source electrode S and the drain electrode D, electrons provided in the channel layer 13 travel through the two-dimensional electron gas layer 15 at high speed toward the drain electrode D. The electrons that move from the source electrode S to the drain electrode D, that is, a drain current can be controlled by changing a thickness of a depletion layer right under the gate electrode G based on a voltage applied to the gate electrode G.
In the HEMT of GaN, it is known that a large electric charge is generated in the channel layer due to a piezoelectric effect, while negative electric charge is generated on the surface of a semiconductor of such as AlGaN. The negative electric charge acts directly on the drain current and greatly affects to a performance of the HEMT. Specifically, when the large negative electric charge is generated on the surface, a current collapse occurs, which degrades a maximum drain current of an alternating current compared with that of a direct current. In order to prevent an occurrence of the current collapse, a surface protection film of silicon nitride (SiNx) has been formed on the surface of the electron supply layer 14.
However, when SiNx is used for the surface protection film for the FET such as the HEMT, there still is a problem that a larger gate leakage current is generated compared with using other type of surface protection film of, for example, silicon dioxide (SiO2). Further, when SiNx is used for a surface protection film for a diode, there is a problem that Schottky leakage current is generated, which is one of the leakage currents generated at a Schottky electrode. Moreover, when SiNx is used for the surface protection film for the diode or the semiconductor devices such as the FET, because a withstanding voltage of SiNx is low, there is a problem that the withstanding voltage of the semiconductor device degrades.